(1) Field of Invention
This invention relates to liquid laundry detergent compositions. More particularly, this invention relates to non-aqueous liquid laundry detergent compositions which are easily pourable and which do not gel when added to water and to the use of these compositions for cleaning soiled fabrics.
(2) Discussion of Prior Art
Liquid nonaqueous heavy duty laundry detergent composition are well known in the art. For instance, compositions of that type may comprise a liquid nonionic surfactant in which are dispersed particles of a builder, as shown for instance in the U.S. Pat. Nos. 4,316,812; 3,630,929; 4,264,466, and British Pat. Nos. 1,205,711, 1,270,040 and 1,600,981.
Liquid detergents are often considered to be more convenient to employ than dry powdered or particulate products and, therefore, have found substantial favor with consumers. They are readily measurable, speedily dissolved in the wash water, capable of being easily applied in concentrated solutions or dispersions to soiled areas on garments to be laundered and are non-dusting, and they usually occupy less storage space. Additionally, the liquid detergents may have incorporated in their formulations materials which could not stand drying operations without deterioration, which materials are often desirably employed in the manufacture of particulate detergent products. Although they are possessed of many advantages over unitary or particulate solid products, liquid detergents often have certain inherent disadvantages too, which have to be overcome to produce acceptable commercial detergent products. Thus, some such products separate out on storage and others separate out on cooling and are not readily redispersed In some cases the product viscosity changes and it becomes either too thick to pour or so thin as to appear watery. Some clear products become cloudy and others gel on standing.
The present inventors have been extensively involved in studying the rheological behavior of nonionic liquid surfactant systems with and without particulate matter suspended therein. Of particular interest has been non-aqueous built laundry liquid detergent compositions and the problems of gelling associated with nonionic surfactants as well as settling of the suspended builder and other laundry additives. These considerations have an impact on, for example, product pourability, dispersibility and stability.
The rheological behavior of the non-aqueous built liquid laundry detergents can be analogized to the rheological behavior of paints in which the suspended builder particles correspond to the inorganic pigment and the nonionic liquid surfactant corresponds to the non-aqueous paint vehicle. For simplicity, in the following discussion, the suspended particles, e.g. detergent builder, will sometimes be referred to as the "pigment."
It is known that one of the major problems with paints and built liquid laundry detergents is their physical stability. This problem stems from the fact that the density of the solid pigment particles is higher than the density of the liquid matrix. Therefore, the particles tend to sediment according to Stoke's law. Two basic solutions exist to solve the sedimentation problem: liquid matrix viscosity and reducing solid particle size.
For instance, it is known that such suspensions can be stabilized against settling by adding inorganic or organic thickening agents or dispersants, such as, for example, very high surface area inorganic materials, e.g. finely divided silica, clays, etc., organic thickeners, such as the cellulose ethers, acrylic and acrylamide polymers, polyelectrolytes, etc. However, such increases in suspension viscosity are naturally limited by the requirement that the liquid suspension be readily pourable and flowable, even at low temperature. Furthermore, these additives do not contribute to the cleaning performance of the formulation.
Grinding to reduce the particle size is more advantageous and provides two major consequences:
1. The pigment specific surface area is increased, and, therefore, particle wetting by the non-aqueous vehicle (liquid nonionic) is proportionately improved.
2. The average distance between pigment particles is reduced with a proportionate increase in particle-to-particle interaction. Each of these effects contributes to increase the rest-gel strength and the suspension yield stress while at the same time, grinding significantly reduces plastic viscosity.
The nonaqueous liquid suspensions of the detergent builders, such as lhe polyphosphate builders, especially sodium tripolyphosphate (TPP) in nonionic surfactant are found to behave, rheologically, substantially according to the Casson equation: EQU .sigma.1/2=.sigma..sub.o 1/2+.eta..infin.1/2.gamma.1/2
where
.gamma. is the shear rate, PA1 .sigma. is the shear stress, PA1 .sigma..sub.o is the yield stress (or yield value), and PA1 .eta..infin. is the "plastic viscosity" (apparent viscosity at infinite shear rate).
The yield stress is the minimum stress necessary to induce a plastic deformation (flow) of the suspension. Thus, visualizing the suspension as a loose network of pigment particles, if the applied stress is lower than the yield stress, the suspension behaves like an elastic gel and no plastic flow will occur. Once the yield stress is overcome, the network breaks at some points and the sample begins to flow, but with a very high apparent viscosity. If the shear stress is much higher than the yield stress, the pigments are partially shear-deflocculated the apparent viscosity decreases. Finally, if the shear stress is much higher than the yield stress value, the pigment particles are completely shear-deflocculated and the apparent viscosity is very low, as if no particle interaction were present.
Therefore, the higher the yield stress of the suspension, the higher the apparent viscosity at low shear rate and the better is the physical stability of the product.
In addition to the problem of settling or phase separation the non-aqueous liquid laundry detergents based on liquid nonionic surfactants suffer from the drawback that the nonionics tend to gel when added to cold water. This is a particularly important problem in the ordinary use of European household automatic washing machines where the user places the laundry detergent composition in a dispensing unit (e.g. a dispensing drawer) of the machine. During the operation of the machine the detergent in the dispenser is subjected to a stream of cold water to transfer it to the main body of wash solution. Especially during the winter months when the detergent composition and water fed to the dispenser are particularly cold, the detergent viscosity increases markedly and a gel forms. As a result some of the composition is not flushed completely off the dispenser during operation of the machine, and a deposit of the composition builds up with repeated wash cycles, eventually requiring the user to flush the dispenser with hot water.
The gelling phenomenon can also be a problem whenever it is desired to carry out washing using cold water as may be recommended for certain synthetic and delicate fabrics or fabrics which can shrink in warm or hot water.
Partial solutions to the gelling problem in aqueous, substantially builder-free compositions have been proposed and include, for example, diluting the liquid nonionic with certain viscosity controlling solvents and gel-inhibiting agents, such as lower alkanols, e.g. ethyl alcohol (see U.S. Pat. No. 3,953,380), alkali metal formates and adipates (see U.S. Pat. No. 4,368,147), hexylene glycol, polyethylene glycol, etc.
In addition, these two patents each disclose the use of up to at most about 2.5% of the lower alkyl (C.sub.1 -C.sub.4) etheric derivatives of the lower (C.sub.2 -C.sub.3) polyols, e.g. ethylene glycol, in these aqueous liquid builder-free detergents in place of a portion of the lower alkanol, e.g. ethanol, as a viscosity control solvent. To similar effect are U.S. Pat. Nos. 4,111,855 and 4,201,686. However, there is no disclosure or suggestion in any of these patents that these compounds, some of which are commercially available under the tradename Cellosolve.RTM., could function effectively as viscosity control and gel-preventing agents for non-aqueous liquid nonionic surfactant compositions, especially such compositions containing suspended builder salts, such as the polyphosphate compounds, and especially particularly such compositions which do not depend on or require the lower alkanol solvents as viscosity control agents.
Furthermore, British Patent Specification No. 1,600,981 mentions that in non-aqueous nonionic detergent compositions containing builders suspended therein with the aid of certain dispersants for the builder, such as finely divided silica and/or polyether group containing compounds having molecular weights of at least 500, it may be advantageous to use mixtures of nonionic surfactants, one of which fulfills a surfactant function and the other of which both fulfills a surfactant function and reduces the pour point of the compositions. The former is exemplified by C.sub.12 -C.sub.15 fatty alcohols with 5 to 15 moles of ethylene and/or propylene oxide per mole.
The other surfactant is exemplified by linear C.sub.6 -C.sub.8 or branched C.sub.8 -C.sub.11 fatty alcohols with 2 to 8 moles ethylene and/or propylene oxide per mole. Again, there is no teaching that these low carbon chain compounds could control the viscosity and prevent gelation of the heavy duty non-aqueous liquid nonionic surfactant compositions with builder suspended in the nonionic liquid surfactant.
It is also known to modify the structure of nonionic surfactants to optimize their resistance to gelling upon contact with water, particularly cold water. As an example of nonionic surfactant modification one particularly successful result has been achieved by acidifying the hydroxyl moiety end group of the nonionic molecule. The advantages of introducing a carboxylic acid at the end of the nonionic include gel inhibition upon dilution; decreasing the nonionic pour point; and formation of an anionic surfactant when neutralized in the washing liquor. Nonionic structure optimization for minimizing gelation is also known, for example, controlling the chain length of the hydrophobic-lipophilic moiety and the number and make-up of alkylene oxide (e.g. ethylene oxide) units of the hydrophilic moiety. For example, it has been found that a C.sub.13 fatty alcohol ethoxylated with 8 moles of ethylene oxide presents only a limited tendency to gel formation.
Nevertheless, still further improvements are desired in the stability, viscosity control and gel inhibition of non-aqueous liquid detergent compositions.
Accordingly, it is an object of the invention to provide non-aqueous liquid laundry detergents which do not gel when contacted with or when added to water, especially cold water.
It is a further object of the invention to provide non-aqueous liquid built laundry detergent compositions which are storage stable, easily pourable and dispersible in cold, warm or hot water.
Another object of this invention is to formulate highly built heavy duty non-aqueous liquid nonionic surfactant laundry detergent compositions which can be poured at all temperatures and which can be repeatedly dispersed from the dispensing unit of European style automatic laundry washing machines without fouling or plugging of the dispenser even during the winter months.
A specific object of this invention is to provide non-gelling, stable, low viscosity suspensions of heavy duty tripolyphosphate built non-aqueous liquid nonionic laundry detergent composition which include an amount of a low molecular weight amphiphilic compound sufficient to decrease the viscosity of the composition in the absence of water and upon contact with cold water.
These and other objects of the invention which will become more apparent from the following detailed description of preferred embodiments are generally provided by adding to the liquid nonionic surfactant composition an amount of a low molecular weight amphiphilic compound, particularly, mono-, di- or tri(lower (C.sub.2 to C.sub.3) alkylene)glycol mono(lower (C.sub.1 to C.sub.5) alkyl)ether, effective to inhibit gelation of the nonionic surfactant in the presence of cold water.